The invention relates to a load cell with a force transducer for recording a weight, the force transducer having a part which does not deform under loading and a force introduction part with an elastically deformable part, the elastically deformable part and the non-deforming part having in a measuring portion a defined spacing in relation to each other which changes under loading, with a sensor arrangement with an inductively operating sensor element, which is disposed in the measuring portion opposite a signaling face, in order to detect changing of the spacing as an electrical signal, and with a circuit for converting the electrical signal into a weighing signal.
Such a load cell is known for example from DE 44 20 691 C1.
Other known force transducers are in many cases based on the deformation being detected as a surface expansion. Typical of these are strain-gage transducers, in which the surface expansion occurring when there is deformation is converted into a change in electrical resistance.
These strain-gage transducers achieve very high degrees of accuracy, but can be protected against ambient influences only with great effort, since the strain-sensitive sensor elements are located directly on the elastically deforming part and covering of the sensors has the concomitant effect of force bypasses, which have direct adverse effects on the properties of the transducer.
In the case of capacitively operating load cells, the elastically deformable part is connected to an electrode of a plate capacitor, deformation of the elastically deformable part caused by the force to be measured producing a change in capacitance.
The inductively operating load cell known from DE 44 20 691 C1 is designed specifically for applications in weighing technology and proves to be unsuitable under extreme ambient conditions, in particular such as occur for example in a motor vehicle when detecting the weight of occupants, since it is too susceptible to interference.
For such an application, a load cell must have the following special properties:
The load cell must have compact outside dimensions, in order to conform to the very confined installation space in the case of such applications. At the same time, it must be mechanically stable, to be able to withstand the very great forces possible in the operation of a motor vehicle.
The load cell must be designed with respect to the sensor element and measured value conversion in such a way that the very high ambient field strengths possible in a motor vehicle do not have any harmful influence on the measuring accuracy. For instance, electrical field strengths of up to 200 V/m can occur in a motor vehicle, an extremely high value in comparison with industrial ambiences of 10 V/m.
The load cell must be capable of operating in a large operating temperature range of from xe2x88x9240xc2x0 C. to +85xc2x0 C. with rapid temperature changes and also possible condensation.
The load cell must be able to withstand very high numbers of stress cycles, as occur in the operation of a vehicle due to the forces of acceleration, without the technical measuring properties changing.
In the course of introducing airbags as standard for the passenger seat in motor vehicles there arises the problem of detecting the occupancy of the seat, so that the passenger airbag is released for firing only in those cases in which the seat is occupied. Firing of the passenger airbag when the passenger seat is not occupied not only has the consequence that an additional increase in pressure in the vehicle is unnecessarily caused in the event of an accident by the firing of the passenger airbag, but also that, due to the integration of the passenger airbag into the dashboard, considerable repair measures are necessary in the passenger compartment of the vehicle following firing of the airbag.
Various problems which have been observed when airbags are fired with small children or youngsters traveling on the passenger seat have ultimately led to the requirement that, in the event of an accident, an airbag must be operated in a way which is adapted to the person sitting on the vehicle seat.
This gives rise to the problem of determining the necessary data for firing the airbag in a way appropriate for requirements.
One possible reference point is to detect the weight of the respective vehicle occupant, the size and physical robustness of the occupant then being concluded on the basis of a weight determination.
The invention therefore relates in particular to a load cell which can be used in ambiences with extreme conditions, for example in a motor vehicle.
In terms of measuring technology, the vehicle interior is extremely problematical, since not only do interference signals of the engine ignition (electromagnetic fields with high field strengths) occur to a considerable extent and have to be eliminated in signaling terms, but it is also necessary with respect to the functional capability of sensors that are to be used to take into account extreme vibrations, temperature conditions in a range from, for example, xe2x88x9240xc2x0 C. to +85xc2x0 C., rapid temperature changes and also atmospheric humidity ranging right up to the dew point.
Furthermore, account must be taken of the fact that not only the weights of the occupants act on the seat but also, when the vehicle accelerates and, in particular, decelerates suddenly, forces which are many times greater, not to mention the forces occurring in an accident situation, which likewise must not lead to malfunctioning of the sensors.
As well as this there are the various adjusting possibilities for a vehicle seat which are provided in modern motor vehicles and may only influence the measuring result within defined limits.
It is therefore an object of the invention to develop a load cell of the type described at the beginning further in such a way that it can be used in particular in conditions which are very difficult in terms of measuring technology, and in particular under the other special ambient conditions within a vehicle when detecting the weight of occupants, and the weighing signal of which is substantially uninfluenced by this.
This object is achieved in the case of the load cell stated at the beginning according to the invention by the force transducer having a recess in the elastically deformable part or the non-deforming part in the region of the measuring portion, by the sensor element being disposed in the recess such that it is shielded against external electromagnetic fields and encapsulated, and by the load cell operating on the eddy current principle.
The load cells operating inductively according to the invention can be constructed in such a way that they are very insusceptible to interference and in particular also very robust, which is of special significance in the case of installation in a seat of a motor vehicle, since great forces can act on the seat even in normal driving operation, during acceleration and deceleration. In the event of an accident, the forces acting on the load cell are many times greater.
On account of the sensor element being disposed in a recess in the measuring portion of the force transducer, protection against mechanical effects on the sensor element is obtained. Since, furthermore, the force transducer is generally produced from metallic material, for example steel or aluminum, shielding against electromagnetic interference fields is also provided as a result.
In spite of the shielding against electromagnetic interference fields, the sensor arrangement is permeable to electromagnetic fields on its side facing the signaling face, dependent on its functional type.
On account of the real part of the measuring signal being evaluated (complex permeability) and the imaginary part ignored, shielding against electrical fields is of no importance. There is no need for shielding intended for this purpose.
The encapsulation allows influences of temperature and atmospheric humidity to be suppressed or even eliminated entirely.
The load cell according to the invention based on the eddy current principle can be constructed in such a way that it is very insusceptible to interference if the sensor effect occurring, in the case of such a load cell, of the change in the real part of the complex permeability of the sensor element is utilized. With respect to the physical effects of this process, you are referred to [Fxc3x6rster, Stambke, Zeitschrift fxc3xcr Metallkunde [Journal for Metallurgy], 4, 45, 1954].
Utilizing this sensor effect allows the sensor element to be used as a frequency-determining element of an oscillator, so that the variable to be measured comprising the weight can be converted into a change in frequency. The output voltage of the oscillator is brought to a relatively high voltage level, of for example about 12 volts, by a downstream level converter.
The advantage of such an arrangement is that the part of the sensor arrangement that is sensitive to external interference fields, that is to say the oscillator circuit and the sensor element (the coil), can be disposed in a very compact and well-shielded manner in the interior of the metallic load cell. The frequency-analogous output signal with a high signal level can then be led to the outside unproblematically, without the risk of the measuring signal being impaired by external electromagnetic interference fields. At the same time, this arrangement also permits easy shielding with respect to other ambient conditions, such as aggressive substances and moisture.
A further advantage of such an arrangement is that such a frequency-analogous output signal can be evaluated by a downstream signal-processing unit very simply, by digital counter circuits that are simple to realize.
A sensor element operating on the eddy current principle is preferably used. Also preferred are those sensor elements in which the weighing signals occur analogously to frequency, since in this case a particularly great and interference-immune signal is obtained. These sensor elements have the further advantage that the weighing signals can be evaluated and further processed in a simple manner.
The recess is preferably disposed in a part of the force transducer which is not deformed. This allows simple cable routing of the signal cables of the load cell and reduces the extent to which the weighing signal is influenced by the latter. The signaling face is then disposed on the elastically deforming part in the region of the measuring portion.
Various solutions for the encapsulation of the sensor element are available.
In the case of a preferred variant, the sensor element disposed in the recess is cast with a curable material.
In the case of another preferred variant, the sensor element is separately encapsulated in a separate housing and is fitted and fixed in the recess together with the housing.
For simple and reliable mounting, even in the case of possibly necessary exchange, the recess is provided with a stop, which allows exact positioning of the sensor element with respect to the distance from the signaling face.
A solution which is simple in production engineering terms is provided by the recess being formed as a through-bore, the housing of the sensor element having a stop for the exact positioning within the bore.
The stop may be, for example, a collar protruding from the housing of the sensor element.
The housing of the sensor element is preferably formed in a substantially cylindrical manner and provided on its cylinder wall with an external thread, the bore of the recess being provided with a complementary internal thread.
The construction of the sensor element itself has so far not been discussed in any detail. It is to be recommended in particular with regard to the aimed-for immunity to interfering factors to construct the sensor element with a ferrite core and a sensor coil disposed in the ferrite core, the ferrite core being formed such that it is open for the passing through of magnetic fields on the side which in the installed state is facing the signaling face. Apart from the shielding of the sensor coil, the measuring effect is also maximized.
The ferrite core may in this case be formed as an open cup core (pot core), as an E core or as a U core.
In the case of a particularly preferred type of construction, the sensor element is disposed as half a pot core in a bore of the force transducer, its magnetic field spreading out in a relatively narrow measuring gap.
A further preferred type of construction of the transducer is that the force transducer, which comprises a non-deforming part fixed to the frame, an elastic deforming part, a force introduction part and also recesses for the sensor arrangement, is to be produced from one piece of a metallic material. This material preferably comprises a curable non-magnetic lightweight metal alloy. In this case, the recess preferably receives, at least partially, not only the sensor element but also the associated sensor electronics.
This has the following advantages:
Curable lightweight metal alloys not only have very good tensile strength, but also withstand high numbers of load stress cycles without impairment. Both are properties which are absolutely necessary for the described use in motor vehicles.
Since such substances are non-magnetizable (the magnetic permeability is around 1), there can also be no subsequent effects on the technical measuring properties. Magnetizable technical materials (steel), which seem appropriate because of their very good mechanical properties, vary greatly in their magnetic properties (to be more precise in their magnetic permeability) for production reasons, leading to subsequent effects on the behavior of the weight sensor which are difficult to control.
Preferably used for the conversion of the deformation occurring when the force transducer is subjected to loading, and the associated change in inductance, into a measuring signal suitable for further evaluation is either a bridge measuring method or else, preferably, an oscillator circuit, which is preferably disposed alongside the sensor element and is encapsulated together with the sensor element in the recess. The sensor element preferably represents a frequency-determining part of the oscillator circuit. This allows the aforementioned advantages to be realized.
In the case of this method, the change in inductance when there is alternating loading of the load cell produces changes in the frequency of the oscillator. This allows a signal which is analogous to the frequency to be generated, with which the further signal transmission can be performed with levels which are very immune to interference (for example TTL). This method also permits very simple further signal processing, since only counters which can be easily read out by commonly used microcontrollers are necessary for the representation of a measured value. Such counters may already be provided in the recess of the force transducer as part of the circuit, preferably integrated into a so-called ASIC chip.
To achieve easily reproducible qualities of the signaling face, it is preferably formed by a composite ferrite-polymer sheet, which is disposed in the measuring portion of the force transducer opposite the sensor element. In the case in which the force transducer is produced from an aluminum alloy, in this case a certain signal gain is also obtained, whereas the advantage of the composite ferrite sheet in the case of force transducers made of steel lies rather in the fact that compensation is provided for the differences frequently observed with this material in the magnetic behavior.
What is more, with the composite ferrite-polymer sheet, the properties of ferrites (high measuring signal) are combined with the easy handling (deformability, adaptability) of plastic sheets.
In order to protect the force transducer from mechanical overloading, the non-deforming part of the force transducer may be formed as a mechanical stop for deformation of the deformable part of the force transducer. If the permissible limit loading of the force transducer is exceeded, the elastically deformable part comes up against the non-deforming part and is supported there. Consequently, excessive loading is avoided, and consequently so too is excessive deforming of the force-introducing part. This measure can be performed in twofold manner, that is to say with regard to expected tensile forces and compressive forces.
In the case of a particularly preferred embodiment, finally, not only the sensor element, and possibly parts of the circuit, but also the sensor arrangement are disposed altogether in the recess. If the entire sensor arrangement is encapsulated in a separate housing, it can be exchanged in an extremely simple way, comparable to a spark plug.
On account of the configuration of the load cells according to the invention, they can be disposed between the seat frame and the supporting frame of a vehicle seat, it being possible to obtain a weight signal which is independent of the position in which the vehicle seat happens to be at the time, seen in the longitudinal direction of the vehicle, and also to allow further adjusting possibilities of the seat, for example height adjustment, setting the inclination of the backrest, pivoting the sitting area about an axis transverse to the longitudinal direction of the vehicle etc., without the measuring signal being falsified to an inadmissible extent as a result.
When the load cells according to the invention are used for weight determination in a vehicle seat, either the seat frame may be connected to the non-deforming part of the force transducer and the supporting frame connected to the elastically deformable part, or else vice versa, the seat frame connected to the elastically deforming part of the force transducer and the supporting frame connected to the non-deforming part of the force transducer, so that the force transducer preferably creates a direct connection between the seat frame and the supporting frame.
In order to realize a relatively high degree of accuracy for the weight measurement, the seat frame is preferably connected to the supporting frame via three or more load cells. Most preferred is an arrangement of four load cells for connecting the seat frame and supporting frame, for example alongside the four corner regions of a sitting area.
The use of three or more load cells in connection with the present invention additionally has the advantage that the force transducers can then be disposed in such a way that the so-called H point (hip point) of the seat position in the vehicle is not increased, or at most is increased insignificantly.
The mounting possibility proposed above as being preferred, that is to say mounting the seat frame on the one hand and the supporting frame on the other hand directly on the non-deforming part of the force transducer and on the elastically deforming part of the force transducer respectively, often even brings the advantage that the H point can be placed slightly lower.
The H point is of interest because it constitutes part of a vehicle registration and is an indirect measure of the headroom of the vehicle. Changing the H point beyond an amount which is fixedxe2x80x94according to the vehicle typexe2x80x94inevitably means that a change which necessitates renewed registration of the entire vehicle is made.
When three or more load cells are used, shifts of the center of gravity of the person sitting on the vehicle seat can in addition also be registered and taken into account in the evaluation of the weighing signals.